Thin thermoplastic films are produced by extrusion, generally by the blown tube method or the slot-die method. In the slot-die method film heated plastic is extruded out of a narrow slot in an elongated die. Flat film is generally made in considerable widths on relatively large machines which run at high production rates of 100 to 300 or more meters/minute. The hot extrudate in the form of a web from the slot-die is drawn and stretched through an air-gap to the desired gauge thickness and width by a roller or rollers which operate at a greater speed than the film is extruded from the slot die. Typically, the film passes from the slot die through an air-gap to a chill roll equipped with an air-knife to assist in maintaining good contact between the film and the chill roll, and finally to a take-up roll.
High pressure low density polyethylene (LDPE) films have been suscessfully produced by the slot-die method at high line speeds. But when linear low density polyethylene (LLDPE) is processed in the same way it has been recognized that it is susceptible to draw resonance at a relatively low draw or take-up rate. Draw resonance is a phenomenon associated with stretching motions and it is manifested by the occurrence of periodic fluctuations in the thickness of the extrudate as it is drawn. Draw resonance is a function of the take-up rate as well as the draw ratio, the draw ratio being the ratio of the die-slot width to the film thickness. For a given linear low density polyethylene, draw resonance is expected to be more severe in drawing a thinner gauge of the film through the same die-gap, and its onset at a lower draw speed, than in a thicker gauge film. Film exhibiting draw resonance can have gauge thickness variations on the order of 5%, or more, which is considered to be commercially unsatisfactory.
Various methods have been proposed for reducing draw resonance such as described in U.S. Pat. No. 4,626,574 in which a short air-gap is used, and U.S. Pat. No. 4,486,377 in which the film is cooled with a gas stream or the like during its passage through the air-gap to reduce draw resonance.
It is recognized in the rheology of polymers that a viscoelastic material behaves primarily as a viscous liquid or an elastic solid depending on the relation of the time scale of the experiment and the time required for the system to respond to either stress or deformation. The relationship has been defined as the dimensions Deborah number, De (see Z. Tadmor and C. G. Gogos, Principles of Polymer Processing, John Wiley and Sons, New York, 1979, pp 40-42, incorporated herein by reference).
A material at high Deborah numbers responds elastically and at low Deborah numbers exhibits mostly viscous behavior. I have found that at high Deborah numbers instability manifesting itself as draw resonance is dominated by elastic deformation and that depression of the elastic deformation by additional heating stabilized the process and reduced draw resonance.
Thus, the solution to the instability problem by heating is opposite to that described in U.S. Pat. No. 4,486,377 in which high temperature operation (515.degree.-575.degree. F., 268.degree.-302.degree. C.) such as melt-embossing and extrusion-coating conducted under low Deborah number conditions, where viscous deformation predominates, is stabilized by cooling the extrudate.